In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter implants a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in “n type” extrinsic material wafers, whereas if “p type” extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway, which will effectively separate ions of different charge-to-mass ratios.
For a batch ion implanter, the ion beam is directed toward an end station that has a number of wafers located around a circumference of a spinning disk or distributed radially in multiple concentric circles. Pads are affixed to the spinning disk at an angle, typically 5 to 10 degrees to the plane of rotation of the disk. The pads hold the wafers in place because of the centrifugal force exerted thereon while the disk is spinning. The ion beam is typically slowly moved across the wafers in a slow scan direction while the wafers pass through the ion beam relatively quickly in a fast scan direction.
However, the tilt of the pads holding the wafers results in a non-uniform ion beam incidence angle that varies according to position of the wafer. The non-uniform ion beam incidence angle can result in undesirable dopant concentrations in a workpiece. In addition, the non-uniform ion beam incidence can cause shadowing artifacts caused by the 3-dimensional photoresist patterns. What is needed are systems and methods that facilitate uniformity in ion beam incidence angles for batch processing systems.